The invention relates to a shoe comprising a sole structure having a top side facing towards a shoe interior space, and an air pump device for blowing air into the shoe interior space, the air pump device comprising a bellows formed in a cavity in a heel area of the sole structure, an intake channel coupled to the bellows for transporting air to the bellows from an intake opening, and an air supply device coupled to the bellows for forwarding air from the bellows into the interior space of the shoe.
Such a shoe is known for example from the documents EP 2 218 348 A1 and WO 2012/126489 A1. In the known shoes of the kind described in the introduction, the sole structure may have a multilayer construction in the heel area, wherein an intermediate layer in which the cavity is located is made from a material (soft polyurethane foam for example) that is intended to be more elastic or more compressible than the material of the outsole. The outsole should be made from abrasion-resistant rubber. The air pump device is designed so that, in alternating manner in response to a walking movement of a user, air is sucked in from outside the shoe via the air intake channel when a load is removed (the shoe is lifted off the ground) and air is blown into the shoe interior space through channels when a load is applied (when the shoe comes into contact with the ground and supports the user's weight). A first valve is arranged in the air intake channel and is designed to allow air to pass only in the direction from outside the sole structure into the air pump device. A second valve is arranged in the air supply device, and is designed to allow air to pass only in the direction from the air pump device to the channels. The pump effect may be enhanced further if the outsole has a raised area on the outer tread in the region of the air pump device, which area is pressed towards the upper part of the sole when the load of the user's foot is placed upon it.
One of the suggestions described in EP 2 218 348 A1 is that the intermediate sole be arranged between a hard outsole and an additional sole, wherein the intermediate sole should be manufactured from a material that is more compressible (more elastic/softer) than that of the outsole and that of the additional sole.
In order to achieve good ventilation of the shoe interior space, that is to say effective airflow, it is essential that during each step when the user is walking a sufficiently large quantity of air is sucked into the bellows from the outside and also that it is then blown out of the bellows and into shoe interior space. In order for the greatest possible quantity of air to be blown into the shoe interior space when the load is applied during each step, not only must the volume of the bellows be maximised; it must also be ensured that when the load is applied the bellows is compressed almost completely, or at least mostly, so that the air it contains is forced out. Complete or substantial compression can be ensured by making the sole structure surrounding the cavity very pliable or soft, so that it is completely compressed by the effect of the user's bodyweight. However, the bellows must also expand and fill with air as completely as possible after the load is removed and before it is applied again (in the next step). Such a recovery is achieved with a sole material surrounding the cavity that is as elastically hard as possible. However, this conflicts with the previously stated requirement that the material be as soft as possible.